Variable displacement compressors are being established as the preferred means for compressing refrigerant in air conditioning systems of present day motor vehicles. The compressor typically includes a plurality of cylinders, each adapted to receive a piston for reciprocal compressing action. As is commonly known, the piston includes a piston head sealed around the inner annular surface of the cylinder, and a piston rod driving the piston head to provide the desired compressive action.
A typical design places a number of radially disposed cylinders equally angularly spaced about the axis of a central drive shaft. A swash plate or wobble plate is received about and is operatively connected to the drive shaft. During operation of the compressor, rotation of the drive shaft imparts a drive movement including radially reciprocating action to the swash plate. The swash plate responds to the drive movement by nutating about the drive shaft, thus imparting a linear reciprocating motion to the individual pistons. This cooperative interaction dictates that the most effective connection between the swash plate and the associated piston rod is a ball joint.
An automotive air conditioner is a unique environment for piston operation. It is advantageous to reduce the size of automotive air conditioning systems as much as possible to reduce the overall weight of the vehicle, to lower the cost and to conserve valuable space. It is also, of course, desirable to improve its operating efficiency. It can be appreciated that substantial momentum forces are capable of being produced due to the extreme accelerations imparted to a compressor piston. Accordingly, there is an additional need to reduce the weight, and thus the mass of the piston itself in order to reduce the forces generated by the piston action. In turn, substantial energy can be saved due to a reduction in the force required to drive the multi-cylinder compressors.
Historically, pistons for swash plate compressors have been of unitary construction, machined from a durable metal body, such as a steel alloy. This prior art construction provides the desired strength, but unfortunately has an undesirably high mass. While improvements in hardening and machining techniques have allowed lighter weight metals, such as aluminum, to be substituted, the unitary construction still dictates that a relatively high mass structure be provided.
In an attempt to provide an improved swash plate compressor piston, some artisans turn their attention away from a unitary or integral construction to providing individual components that are assembled in an attempt to provide an improved structure. An example of such a design is disclosed in U.S. Pat. No. 3,354,791 to Wahlmark, issued Nov. 28, 1967. The piston there disclosed is constructed of separate piston head, shank and ball components. The shank is fabricated from a strip of spring steel rolled into a hollow tube. The fabrication process of the shank creates a seam that allows the shank to be compressed radially. This allows the shank to be inserted into a bore machined within separate head and ball components, both being made of a high strength steel alloy. The complete piston is relatively light-weight since the shank (piston rod) is hollow.
While this design provides a simple and inexpensive approach to a piston assembly of equivalent strength and durability as compared to the standard unitary design, further improvement is needed. More specifically, the generally high masses associated with metals creates a need for use of alternative materials. However, it should be appreciated that strength and durability cannot be compromised and, thus, an optimum design is desired to provide enhanced strength to the piston assembly at critical points, including the piston rod. The new approach must assure that the entire piston assembly when subjected to the substantial stresses generated during the compressing action is not subject to failure.